This review has been
something like six years in the making. I've been a long-time fan of transformer
volume controls (TVCs); I've been using a home-brewed S&B transformer preamp
since 2003. What I was missing was not the fantastic sound transformers can
give; it was the remote control functions I wanted. John Chapman, a.k.a. Bent
Audio, sold control modules and chassis for TVCs for years. When S&B decided
to move away from supplying the DIY market, John upgraded to new transformers
but needed to find the time to develop a new chassis to match his modules. It's
taken a while (six years!) but John's been busy with all sorts in between.

John uses a particular type of transformer called an
autoformer, this has a single winding; they do not have primary and secondary
windings as normally found with transformers. So instead of TVC we have AVC --
Autoformer Volume Control. The AVCs John uses are from the highly respected
designer, Dave Slagle. John has rather neatly named his relay switched module
with Dave's AVC at its heart the "Slagleformer". Even though AVCs can use a
smaller core than a traditional transformer the Slagleformer uses a core of
similar size to most TVCs. The Slagleformers are optimized for core size, signal
level handling, and bandwidth. The original design has stood the test of time,
not needing any revisions. Right first time is always best! A closely associated
Dave Slagle AVC to the ones John Chapman is using was reviewed
for Enjoy the Music.com by Dick Olsher. What Dick reviewed was a
very affordable hair-shirt volume control kit which ultimately sounds very
similar to the AVC-1 but with coarser volume control steps and no remote
options. John Chapman developed the switching arrangement for Dave Slagle. You
can find more details about the cost effective AVC at Dave Slagle's Intact
Audio site.

What's
all the fuss about using TVCs and AVCs for volume controls? For me they are
synonymous with uncompressed dynamics, resolution and imaging. Most volume
controls are resistive; when you turn down the volume a proportion of the
voltage and current is thrown away
along with dynamic contrasts. One of the reasons often given for transformers
sounding for good at low volume as well as high volume is that they only
attenuate the voltage, not the current. With AVCs more potential current becomes
available as you turn down the volume. Nothing is perfect in this hi-fi lark so
what are the downsides? The inductive load transformers present can cause
problems for a very few output high impedance tube phono stage designs but these
types are intended to feed into active preamps in the first place. In truth they
would fare worse into a typical resistive passive preamp. The vast majority of
tube sources will drive the Slagleformers with ease. Any solid-state phono stage
or CD player should drive Slagleformers with supreme ease too. Here's the
reason; AVC's lower the impedance seen by the source. With the level turned down
to a typical setting, the source(s) are able to drive the power amplifiers
vastly better than if they were directly connected to the amplifiers. The
Slagleformers can be thought of an impedance helper.

Before I'm jumped upon for calling the AVC-1 a pre-amplifier
when it's a passive device I should point out that it will provide 7dB of gain
so it can amplify. Some folks get picky about passive devices being called
amplifiers when they don't amplify – well in this case they can amplify, so
there!

SpecificationThe
AVC-1 is not a hard-core kit and it's not a finished product either. It is a set
of modular options and a chassis which you can assemble into a preamp according
to your required specification. All the modules are pre-populated with
components. Should you want to include your own active modules within the
chassis there is plenty of room, likewise it can be built as balanced too. The
combination of modules I chose is what I would expect to be the most commonly
requested specification:

Single-end (balanced can be spec'ed and requires 4
Slagleformers)

6 pairs of RCA inputs

2 pairs of RCA outputs

Remote control of input selection, volume and balance, all relay operated

Volume control from -54dB to +7dB in 1dB steps
Numeric display of left & right volume settings

LED indication for mute, selected input, t

ape

Stealth mode option to turn off the entire display

Once you unpack your prized delivery, spend plenty of time
understanding what everything does and how to complete the assembly. The
extremely well executed circuit boards for the relay operated input and control
sections need to be bolted to the chassis as does the small control power supply
board. You then have the all-important Slagleformers. These are top and tailed
with circuit boards providing really slick relay switching and they also
eliminate virtually all wiring. Typically these sorts of volume controls have
wires flying everywhere and there are many wires to prep plus solder too. Not
here! All that needs to be done once the modules are in place is to prepare and
solder seven wires in place. The wiring taken at an easy pace took me around 2.5
hours to complete. It might sound like a long time but I wanted to do a decent
job. I used OFC magnet wire, you can use whatever you favorite wire is. The cost
of the chassis, modules, and remote is $1600. You can choose to invest $1300,
yet this comes without the chassis should you decide to use your own box.

The AVC-1 chassis designed by John Chapman is sourced from a
chassis supplier to several well-known brands. It's a quality chassis; it it' or
$1, been something like 6s all aluminum too which is great for sonics. The
chassis is solid without being crazily hewn out of a solid aluminum billet. John's
struck the perfect balance between heft and affordability. The fastenings,
control knob, buttons and phono sockets all have a really good fit and finish.
Then there's the remote control….John went totally over the top with this. He
tells me he couldn't find a suitable off-the-shelf item so he designed the
extruded case, chromed end-plates, circuit board and chromed buttons. The result
is a remote control which would not disgrace a $20,000 preamp. It weighs half a
pound and is brilliant to hold and use. The remote is neatly named the Bent
Metal Handset – it's anything but bent. Seeing as you will be touching the
handset when operating the AVC-1 it seems John has used thinking from the
automotive industry. What I mean is that high-end vehicle brands focus on the
parts you touch to enhance the feeling of quality, i.e. door handles and
controls all need to feel good when operated if you are to feel good about the
whole vehicle.

PerformanceAs
I had previously borrowed an Intact
Audio AVC from a friend I knew what sound to expect from the
AVC-1 and I wasn't disappointed. Borrowing the low cost AVC was valuable as I
didn't want to move to a nicely remote controlled AVC which didn't quite match
up sonically to my S&B TX-102mII TVC. Both Intact Audio AVC and the Bent
Audio AVC-1 at a minimum match the TX-102 and even out-perform it in some
aspects. I should set some context about the S&B TX-102; the successors to
these are found in the Music First Audio preamp. TX-102 was available as a DIY
component in mk I, II and III forms. I'm advised the II and III were the more
consistent sounding meaning that they behaved well with the widest range of
systems. Arguably my mkII was the most truthful sounding version, though some
like the extra top-end with the mkIII.

The AVC-1 sounded great from initial switch on, it does
improve with a hundred or so hours use, getting smoother over time though it
never sounded remotely harsh to me. Used within the context high quality system
you will hear what a great preamp the AVC-1 is. My system comprises 2 record
decks (Garrard 301 and Trans-Fi Salvation), Computer Audio with various DACs,
300B SE power amps feeding Open Baffle speakers (hybrid Bastanis / Trans-Fi). I
also brought a couple of T-amps and Valvet Er1 SE solid-state power amps into
the mix. With Open Baffle speakers including two actively driven and DSP'ed 15"
bass drivers per side I'm easily able to pick up on spatial clues and as for
bass; if it's deep, tuneful and powerful I can easily hear it -- and I do with
the AVC-1. Operation is faultless; the remote is especially wonderful and the
relay operation of inputs and level is really slick!

In
doing a straight comparison it was clear the AVC-1 shared most traits with the
well renowned TX-102. Oodles of resolution, excellent imaging and spatial
effects, silent noise floor, subtlety and almost visible texture. Dynamics...
AVCs should preserve dynamics valiantly and do so at any volume. Yes, dynamics
with the AVC-1 are every bit as good as you'd hope for and probably better than
most people are used to. So what does it do better? The area which struck me
immediately was the treble. It is more extended, that's not to say brighter
because it isn't. The treble simply extends further upwards. In harmony with
this is significantly better decay of high-frequency sounds. One of my favourite
tracks for testing bandwidth and treble balance is Jennifer Warnes "Bird on a
Wire". The tinkly treble in the background can on some systems be a little
recessed, for me it's quite obvious when the balance is correct and it is with
the AVC-1.

The AVC-1 costs $1600, this provides you all the parts to DIY
build a high-end sounding preamp with excellent functionality and appearance.
Unless you need a preamp that warms up a cool sounding system or need high gain
then I can't think of a more ideal preamp, what more could you want?

Manufacturer Comment

Many
thanks to Clive for his endless patience while I sourced a new case option for
DIY builders. For many years now folks have been building projects using these
same internal modules but often they were stuck for what to do for a chassis. It
is nice to now have a chassis option for them. Also I'm glad that Clive found
the DIY part of the project easy - the pre-assembled and tested modules were
made with just that in mind. He's an experienced builder but the project should
be fun for anyone with a bit of DIY experience. Finally (and most
importantly!) it's great to hear that he found the pre-amp musically satisfying.
Credit for that performance must go to Dave Slagle as it's his AVCs that are the
sonic heart of the AVC-1